Split modules via wildcard

Problem/Motivation

This may be a use case specific to my setup, but we use a upstream repo in Pantheon with config split. This repo is merged into the site repo when changes to either are made.

To manage config specific to custom modules in certain sites, it's necessary to define the module names in the upstream config split entity and switch them on via settings.php.

I'd like to be able to define wildcard modules in config_split, so the lookup and split can be done depending on the available modules on each site.

Proposed resolution

I have created a PoC that addss a new field in the config split form "Additional modules by wildcard (*)"

This then uses the ExtensionDiscovery service.

Remaining tasks

This is exploratory development to see if this is useful to the wider community. For my use case, the following tasks remain:

User interface changes

New field in config split entity form as explained above.

Data model changes

No changes. Config split entity schema is unchanged.